1. Field of the Invention
The present invention relates generally to a method and apparatus for modulation of broadband optical signals, and more particularly to a method and apparatus for combining interleaved optical single sidebands with a modulated optical carrier.
2. Description of Related Art and General Background
Conventional optical fiber transmission systems, such as optical fiber community access television (“CATV”) transmission systems can carry multiple channels on a single optical fiber communication line. The channels are transmitted modulated on a wideband signal made up of a plurality of frequency division multiplexed carriers. A wideband optical detector or photo-receiver receives the wideband signal. Each individual channel can be recovered by a heterodyne tuner along with an appropriate microwave filter. An optical fiber transmission system using this type of modulation technique can transmit analog or digital signals and is known as a sub-carrier multiplexed (“SCM”) optical transmission system. FIG. 1 shows a schematic diagram of a typical SCM system which is described in detail in W. I. Way, Subcarrier Multiplexed Lightwave Systems for Subscriber Loop Applications, Journal of Lightwave Technology, 1988, pp. 1806–1818.
High spectral efficiency digital modems may be used to greatly increase the spectral efficiency of conventional SCM techniques. For example, an optical transmitter with a 1 GHz bandwidth can transmit 166 sub-carrier 6 MHz 64-QAM (quadrature amplitude modulation) channels. Since each channel can carry 30 Mb/s of data, 4.98 Gb/s of data may be transmitted, which gives a spectral efficiency of approximately 5 bits/sec/Hz. In comparison, the same transmitter can transmit only 1.4 Gb/s of on-off keying data for a spectral efficiency of only about 1.4 bits/sec/Hz.
There are two important problems to overcome when using a broadband optical transmitter to transport a large quantity of digital data using SCM technology. The first is that the receiver must be a very wideband photoreceiver, which tend to have high spectral noise density and require a complicated and expensive heterodyne receiver. The second is that SCM is an optical double-sideband modulation (ODSB) technique, as shown in FIG. 2A. This means that half of the bandwidth is wasted, as each of the upper and lower sidebands are carrying the same information. One solution to this problem, as shown in Olshansky (U.S. Pat. No. 5,301,058), is to eliminate the lower side band to produce an optical single-sideband signal (OSSB) as shown in FIG. 2B. One may then combine many OSSB modulators, using multiple carrier signals, to more efficiently use the available optical fiber transmission spectrum. This is illustrated in FIG. 2C. This is known as OSSB-DWDM, or optical single side band, dense wavelength division multiplexing. Using double OSSB (D-OSSB), the upper and lower sidebands carry different signals, as shown in FIG. 2D. Thus, the required number of carriers is only half of that required by the OSSB modulation shown in FIG. 2C.
When amplifying the transmitted signal in a conventional multiplexing method, the carrier signal is likewise amplified. Amplification of the carrier signal represents a waste of amplifier gain, since gain is used to amplify a signal that carries no information. Moreover, as power density in the transmission fiber is increased, signal distortions due to optical nonlinear effects are also increased. Elimination of the carrier signal can significantly decrease the total signal power, thereby reducing the total power density and nonlinear effects.
One method for suppressing the carrier is disclosed by Olshansky (U.S. Pat. No. 5,301,058) and Price (U.S. Pat. No. 6,118,566). However, the method requires a pair of Mach Zehnder interferometers and a pair of microwave modulators to generate just two sidebands. The apparatus is complicated and costly.
Yet another method for suppressing the carrier signal is disclosed by Jopson (U.S. Pat. No. 5,745,273). Jopson makes use of a dual path modulator arranged in an optical loop. The light is divided by a coupler which provides a portion of the signal to an optical fiber traveling in each direction around the loop. The signal in one direction is modulated to create a carrier and sidebands while the other is solely the carrier. Upon recombining the two optical signals in a combiner, a signal is produced in which the two carrier signals cancel each other and leave only the modulated signal. One drawback of the Jopson arrangement is the requirement of extremely strict tolerances with respect to the lengths of the paths of the loop so that the two signals will arrive at the combiner having the carrier signals exactly out of phase. This requirement makes the Jopson device difficult to implement in practice.
Another important fact is that the suppressed optical carrier implies a waste of optical power. Therefore, it is preferred to re-use the optical carrier by modulating it with a new baseband data channel.
Even these solutions are imperfect. Use of an OSSB suppressed carrier (OSSB-SC) modulation method, when applied to multi-channel, long-distance optical fiber transmission systems, presents three additional problems. First, conventional narrowband optical filters have a slow roll-off which makes it likely that an optical filter used in the receiver will allow portions of adjacent channels to enter into the filtered window, producing noise in the signal, as illustrated in FIGS. 3A and 3B. Second, as illustrated in FIG. 3B, residual images are produced due to imperfections in the 90° phase shift of the high frequency electrical modulating signal or in the phase shift of the optical signal between the arms of the Mach-Zehnder modulator. Third, dispersion causes self- and external phase modulations which tend to produce distortions in signals transmitted over long distances at 1550 nm, due to beating among the several optical channels, e.g., four-wave mixing products. This last problem may be reduced by the use of conventional dispersion reduction techniques such as use of a chirped fiber grating or dispersion compensating fibers. However, both of these conventional techniques are costly and cannot manage the entire wavelength range.
To avoid residual images and optical nonlinearity-induced distortions, there is a need to use interleaved optical single sidebands, or optical single sidebands having unequal spacing between neighboring channels. To suppress the optical carrier and yet still re-use it, there is a need for an optical carrier notch filter that combines interleaved optical single sidebands with a modulated optical carrier.